The present invention relates to a thermosensitive image transfer recording medium for use with printers for computers and word processors, and bar code printers, and more particularly to a thermosensitive image transfer recording medium from which images are thermally transferred onto a transfer sheet by utilizing the thermofusibility of a thermofusible ink layer thereof.
Conventionally, there is widely known the thermosensitive image transfer recording method as a convenient method of recording images on a sheet of plain paper. This thermosensitive image transfer recording is carried out in such a manner that a thermofusible ink layer of the thermosensitive image transfer recording medium is melted with application of heat by a thermal head and an ink composition of the thermofusible ink layer is imagewise transferred to a transfer sheet, which is superimposed on the thermosensitive image transfer recording medium, opposite to the thermal head with respect to the recording medium.
The above-mentioned thermosensitive image transfer recording medium, generally called an ink ribbon, basically comprises a support and a thermofusible ink layer formed thereon. More specifically, a representative thermosensitive image transfer recording media is constructed in such a fashion that a thermofusible ink layer comprising as the main components a coloring agent, a thermofusible wax component such as waxes, and/or a binder agent such as a resin having a low-melting point (hereinafter referred to as a thermofusible resin) is formed on a support.
Another representative thermosensitive image transfer recording medium is constructed by interposing a release layer comprising as the main component a wax between the support and the thermofusible ink layer. This thermosensitive image transfer recording medium has an advantage in that an ink component contained in the thermofusible ink layer is fused and transferred onto the transfer sheet smoothly. However, even though the release layer is provided in the conventional thermosensitive image transfer recording medium, the image quality obtained by the above conventional thermosensitive image transfer recording medium is greatly influenced by the surface smoothness of the transfer sheet, so that it is difficult to obtain high quality images on the transfer sheet, for example, on a bond paper which has a low surface smoothness, by the conventional thermosensitive image transfer recording medium.
For the purpose of mitigating the above-mentioned shortcomings, a variety of proposals have been made. For example, heat treatment is performed after images are transferred to the transfer sheet as disclosed in Japanese Laid-Open Patent Application 58-76276; an auxiliary means of improving the image quality is taken in the course of the image transfer by using magnetic force, as disclosed in Japanese Laid-Open Patent Application 52-96549, or by using electrostatic force as disclosed in Japanese Laid-Open Patent Application 55-65590; an oily material is added to the thermofusible ink layer to reduce the melting viscosity thereof as disclosed in Japanese Laid-Open Patent Application 60-25762; and a thermal-decomposable material is added to the thermofusible ink layer for thermal sensitization of the ink layer as disclosed in Japanese Laid-Open Patent Application 60-82389, or a thermalexpansible material is added to the thermofusible ink layer for thermal sensitization of the ink layer as disclosed in Japanese Laid-Open Patent Application 60-25762.
Furthermore, there has been proposed a variety of the thermosensitive image transfer recording media which comprise a multi-layered type thermofusible ink layer in order to improve the image quality. For instance, lamination of two thermofusible ink layers is disclosed in Japanese Laid-Open Patent Application 59-224392. Each of the above-mentioned two thermofusible ink layers comprises an individual thermofusible ink composition having a slightly different melting point and each or both of them comprise a pigment. A further thermosensitive image transfer recording medium is disclosed in Japanese Laid-Open Patent Application 60-97888, which comprises a thermofusible material layer comprising a thermofusible material, but comprising no coloring agent, is overlaid on a thermofusible ink layer.
However, the method of melting the ink composition and transferring the melted liquid-type ink composition onto a transfer sheet for image recording has the shortcoming that the image quality on a transfer sheet having a low surface smoothness is inferior to that on a transfer sheet having a high surface smoothness. This fundamental shortcoming that the image quality depends on the surface smoothness of the transfer sheet cannot be eliminated by the conventional thermosensitive image transfer recording media.
As a method of eliminating the above-mentioned shortcoming, there has been proposed a thermosensitive image transfer recording medium comprising a thermofusible ink layer which comprises an ink composition having as the main component a resin, which becomes viscid with application of heat energy thereto, but exhibits mechanical strength to some extent, without becoming a low-viscosity liquid. In the case of this thermosensitive image transfer recording medium, even when images are transferred onto a transfer sheet having a low surface smoothness, the above-mentioned ink composition contained in the thermofusible ink layer adheres to the convex portions of the low-surface-smoothness transfer sheet, covering the concave portions thereof, and accordingly high-quality images can be obtained on the low-surface-smoothness transfer sheet.
However, the above ink composition comprising as the main component such a resin requires more thermal energy for melting the resin and transferring images onto the transfer sheet, in comparison with the conventional ink composition comprising as the main component a wax. Therefore, when the above-mentioned thermosensitive image transfer recording medium comprising a resin-type ink composition is employed, it is necessary to use a film having excellent heat resistance as a support and there are problems that the life of a thermal head is shortened and the built-up heat in the thermal head degrades the image quality.
Furthermore, there is known a conventional thermosensitive image transfer recording medium comprising a support and a thermofusible ink layer, formed on the support, which comprises as the main components a thermofusible wax component such as paraffin wax and a coloring agent such as a dye and a pigment. In this conventional thermosensitive image transfer recording medium, the mechanical strength of the thermofusible wax component is so poor that the abrasion resistance of the transferred image is insufficient for use in practice.
To solve the above problem, a low-melting resin is added to the thermofusible ink layer. As the amount of the resin component is increased, the adhesive force of the thermofusible ink layer to the support is strengthened, but this is disadvantageously accompanied by the difficulty in the transfer of the ink components of the ink layer from the support. In addition to the above, the more the amount of the resin component in the thermofusible ink layer, the lower the thermosensitivity of the thermofusible ink layer. As another countermeasure, there is a trial of making the thermofusible ink layer as thin as possible. However, this makes the transferred images less uniform and the image density thereof lower.
To lower the adhesive strength of the thermofusible ink layer to the support even when the amount of the resin component is increased in the thermofusible ink layer, it is proposed that a water- or solvent-dispersed powdery ink component be contained in a thermofusible ink layer. This thermosensitive image transfer recording medium, however, has the shortcoming that the thermosensitivity of the thermofusible ink layer is degraded.
Occasionally, the images transferred to a transfer sheet by using a thermosensitive image transfer recording sheet are prone to become excessively glossy. Therefore various proposals have been made to roughen the surface of the images to obtain mat images.
For example,
(i) A thermosensitive image transfer recording sheet in which a thermofusible ink layer is formed on a support whose surface is made rough. PA1 (ii) A thermosensitive image transfer recording sheet in which a mat layer having a roughened surface and a thermofusible ink layer are successively overlaid on a support.
By the thermosensitive image transfer recording sheet (i), however, the object of decreasing the glossiness of transferred images cannot be achieved satisfactorily.
In the thermosensitive image transfer recording sheet (ii), the surface of the mat layer is roughened by the addition of silica thereto. When the amount of silica is increased to sufficiently decrease the surface glossiness of the ink component formed on the transfer sheet, the adhesive force of the mat layer to the support is so decreased that the thermofusible ink layer tends to be transferred to a transfer sheet together with the mat layer.